Process for the manufacture of chromates and bichromates



fizz-211st; 6, 1924.

UNITED STATES- PATENT 0FFlCE.

PROCESS FOR THE MANUFACTURE OF CHROMATES AND BICK ROMATES.

R0 Drawing.

To all whom it may concern.

Be it known that we, Anotrnn Jouvn and AN RE HELBRONNER, citizens of the Republie of France, residing at Paris, Department of the Seine, in France, and havin l. 0. addresses, the first 1 Boulevard aint-Germain and the second 79 'Boulevard Males herbes, in the said city, have invented certain new and useful Improvements in a Process for the Manufacture of Chromates and Biehromates; and *WB'CiO hereby declare the following-to he a full, clear, and exact description of the invention; such as will enable others skilledin the art to which it appertainsto make and use the same.

This in ention has for its object improvements in processes for obt'ainin alkalLehromates and bichromates electro ytically;

The electrolvtic manufacture of alkali chromates has lii-therto iven rise to serious errors owing to, the met od used, this being the electrolysis of metallic chromium or of a chromiumallo with an electrolyte formed 0 alkali sa t of a fixed acid.

recesses tend to produce chromic acidgsettmg free at the same time the fixed acidsof the alkali saltsand these dillereut 'acids -would require subsequently, in the course of the manufacture, to be neutralized by a hydroxide composition added from time to time. In like-manner and during vthe operation, the metal alloyed with the chromium isflpreeipitated byfthis addition of hydroxide;

The process whichforms'tl esubject matter of the resent invention consists in electrolyzing as the ferroch1'omiums)-= in anelectrolyte of alkali carbonate, such as mdinm carbonate. As a result of the reactions, the chromic acid which isformcd is immediately converted into a. stable alkali chro'mate' F urthermore there can-he lo free acidity arising from thefixed acids.

The acidity dde to CO disappears -immediately, :this' compound being evolved in art.- in the free state while 'the other part is saturated by the oxide offiron which is produced-during the electrolysis of the folrochromitilm' f v The addition of hydroxide in the present process is thereforeunnecessary. In mality' the neutralization and the prccipitation of the oxide of iron are efi'ected by virtue of the nature of the-electrdlyte which, the whole of the operation, remains condirectly a ehromium 'alloy (such into bich romatei up Application filed March 14, 1922. Serial No. 543,769.

stantly alkaline and never presents any trace of acidity. The electrolysis is carried out without the use of diaphragms. Besides the electrical resistance of the 'diaphra in which is thus dispensed 'with, there is a so avoided the very. rapid warping which would i be due to the oxide of iron penetrating into the pores of the said diaphragm, thus renderin inefl'ective all cleaningyhoweverfrequent y it may be done, and soon rendering the apparatus useless.

aluminium and lime by the decomposition of the material of which the diaphra m is com- Finally there canno longer be any formation of c romates of of'the chromates proceeds uninterrupteilly,

until either the electrolyte or the electrode containing the choromium is exhausted, with a constant increase in the concentta tion of the chromate'.

It also permits:' i, 1. Of eli'ecting' the electrolysis in aconstantl alkaline medium. 2? f avoidingan addition of alkaline hydroxide or of lime dnring the elec-' trolysis.

3. Of-otitaiiiing Qthe'automat ic and we stant precipitation of the iron, in the form of oxide or carbonate, eliminating this product proportion;

q 4. Of preventing the adhesion of ferric deposits, both by the fact of the tempera ture of the electrolyte and by the evolution of the carbonic acid produced.

5; Of "avoiding" the separation subsequently of any deposit, such as sulphate of lime';which would be produced 10 the saturation of the acidity fixed by the ime in the other processes. 4

Experience has shewn' that i hen allthe v carbonate has disappeared in consequence of its transformation into chromate, the elec trolysis neverthelesscontinues to go oirregu larly. This is because the chromate then.- acts as an electrolyteexactly as the carbonate did previously. A second molecule of chromium thus bOCO'I'HQS dl SSOIVGd and theneutral cliromate is converted progressively to the integral transformation into lgjchroniate. It has been found thatin the production of bichromates, the concentration in bichro- -dm'ing mate formed has noactiongbn the progress of the electrolysis, and tl ere fone o n. the forfoo tain chromate and not bichrornate, theelectrolysis will be pushed forward until hichromate is formed and neutral chromate then reverted to, by direct saturation of the concentrated solution of bichromate by a neutral alkali carbonate.

-It is unnecessary in this case to trouble about the quantity of carbonate to be added, as it has been seen that an excess of carbonate will not dissolve in very concentrated solutions of alkali chromate.

The above explanation shows that it is possible to install a process of manufacture of alkali bichromates, in a direct manner and in a single operation by anodic electrolysis of ferrochromiums in an alkaline electrolyte, preferably,in concentrated solutions of carbonate. The concentration may be effected during the electrolysis itself, by readdin carbonate until the liquid, after its trans ormation into bichromate, becomes of a concentration near that at which it crystallizes by cooling down. In practice it is not necessary, however, to push this concentration as far as crystallization in order 1:0 be able to first separate the oxide of iron, 'aby filtration for example; a slight concentration is then sufiicient by ap Jlication of heat in order to obtain crysta lization or even solidification.

The products obtained, i. e. bichromates and'subsidiarily neutral chromates, are rig ourously exempt from iron and such alkali salts as carbonates.

The installation for carrying out the process, comprises substantially a series of clcctrolyzing tanks containing alternate series of anodes of cast ferrochromium and cathodes of lead for example. The liquid circulates from one tank to the other, in which it is subjected to the action of the current. In view of the large number of amperes (seve al hundred ampcres per square metre) which it is possible to use, the electrolyte heats up rapidly, the operation therefore taking place in the presence of heat.

As a practical example of the a plication of the method, an electrolytic cel was employed, with. two anodes, each approximately 30 cm X 62 cm X 5 cm consisting of 61.7% Cr, 33.1% Fe and 5.2% Si and C,

with two lead cathodes of corresponding Size. immersed .in a bath of 304 liters of sodium carbonate solution containing 169.3 gr. pmlitr-r.

The rlectrolyzation was carried out in chromate.

twelve stages or periods, as indicated in the following table, which indicates the time intervals and the average ampera e and voltage of the current employed uring each During period 1 to 4 inclusive, sodium chm mate was formed and from periods 5 to 7 the chromate was changed to bi-chrom'ate. At this stage the operation was stopped in order to filter the liquid, and 30 liters of the filtrate was taken fo the purpose of extracting the crystals stages 7 to 12 bichi bniate of sodium was continuous] obtained b electrolyzation of the residue filtrate whic had been returned to the apparatus, solid carbonate of soda being added regularly during the seventh stage, but the'quantity of the addition being not sufficient to efl'ect the production of the When the o ration was suspended at the end of the twelfth stage, the

cell contained approximately 140 liters of 10 mud, the liquid pro ortion-of which contained 577 grams per iter of sodium-bi-chromate with 2 molecules of water, which was readily crystallized by cooling.

Lar e forme an although this has no influence of itself on the electrolysis, it is well to avoid too great an accumulation thereof. The oxide of iron will therefore be separated from time totime in the course of the operation, either b agitating and then filtering the liquid which is returned to the tanks, or it may be removed by centrifugal action.

Instead of using anodes of ferrochromium fused in the electric furnace, chromite may be mixed with carbon and an agglomeran't such as itch and the briquettes thus obtained su mitted to av temperature sufficient to produce the total, or at least the partial reduction of the oxide of chromium, this re- 12o duction" being effected by fire, without reaching the point of fusion, and furnishing a conglomerate which will be suitable for use as anodes. I

Finally, experience has also shown that the preceding electrolysis may be carried out by alternating currents instead of continuouscurrents, provided. that the density of the amperage used for the reaction be extremely high. In this way all the advanevaporation. From 90 uantities' of oxide of iron are 105 tages of the alternating current may be profited by.

Claims:

1. The method of producing a chromium salt of an alkali which comprises electrolyzing a solution of an alkali carbonate with an anode containing substantial proportions of chromium.

2. The method of producing chromates of alkali metals which comprises elcctrolyzin a solution of the carbonate of the desired alkali metal with an anode containing chromium in substantial proportions.

3. The method of producing chromates and bichromates of alkali metals which comprises electrolyzi'n a solution of the carbonate with an ano c containing substantial roportions of chromium to first form al- 'al1 chromatc, and continuing the eiectrolytic action to convert the resultant chromate into bichromate.

ADOLPHE J OUVE. ANDRE HELBRONNER. 

